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K3 Enhancements and Mods and Tech Alerts

All topics in this section apply to the Elecraft K3 model only and have been incorporated into all K3S models now shipping.  That is, if you own a K3S model radio, you already have all of the items listed below incorporated into your radio.

How to Use this Section

The following performance enhancements have been incorporated into production K3 radios in response to customer requests or as part of ongoing engineering improvements. Any change that was made after a particular K3 was shipped can be added later, if desired. All changes are optional, and some apply only to specific operating conditions or configurations.

To determine which changes are applicable to your K3, check the Production phase-in date in the table below. If you were close to the cutoff, refer to How do I determine if I already have this?

The reason for each change is provided in the description column. Additional details can be obtained by clicking the document name under Enhancement / Modification. These enhancement parts kits for most mods are available on our K3 parts and mod kits order page.

If your K3 has returned to the factory

If your K3 has recently (last 5 years) returned to the factory for option installation, repairs or calibration, many of the enhancements will have been applied to your radio.  Please check the Repair Report that accompanied the K3 when it returned to you for which ones have already been installed in your radio.


All K3 radios will have a serial number below 10,000. The Enhancements and Modifications found on this page applies to K3 radios below s/n 10,000. Any K3 returned for repair, upgrades or option installation will have many of these mods applied prior to the radio being returned to its owner. Please check the Repair Reports for which modifications have been applied. All K3S radios (above serial number 10,000) have had these circuit changes incorporated into the design and are not applicable to any K3S.

 Enhancement / Modification

Click for Document


Production  phase-in shipping date

(Dates and serial numbers are approximate and may vary slightly.)

K3EXREF+K144REFLK Frequency Shift Fix


The K3EXREF module (which locks the K3 to an external 10 MHz reference) may cause a small, repetitive frequency shift of the 49.38-MHz reference oscillator. This is generally noticeable only on 2 meters, when a K144XV module with PLL lock option is also installed. The app note describes a simple modification requiring two small jumpers on the KREF3 reference oscillator module.

This modification is recommended.



KSYN3 DDS Gain Modification


In rare cases, at certain frequencies the DDS driving signal to the Phase Locked Loop (PLL) may be marginal, resulting in a sudden shift in frequency (commonly called “chirp”). This modification adds a resistor to the KSYN3 board that increases the DDS drive level prevent this condition.

This is an optional modification and is not required unless you are experiencing this condition.


Approximately K3 s/n 3922 and later

IF Output Buffer Gain Modification

This modification increases the signal level at the I.F. output by more than 10 dB.

It requires changing one easily-accessible resistor on the K3’s main RF board with either another surface mount device (SMD) or a leaded 1/8 watt resistor.

A kit containing both an SMD and leaded resistor is available from Elecraft: K3IOBUFFKT.

About 09/01/09 (RF board Rev H3)  and later.

K3 VFO Noise Mod, Rev C.

On some K3s weak VFO tuning noise is heard on a few frequencies in the 60M, 10M and 6M bands (usually only with the antenna removed). This application note describes a simple mod to eliminate or minimize this noise. This mod also requires new K3 f/w, mcu 3.25 or later.

About 09/01/09 (RF board Rev H3)  and later.

K3 Extreme Signal RX Protection Mod.

This 1 wire mod provides additional extreme RX signal protection for the K3 by automatically turning off the pre-amp (and turning on the attenuator, if needed) when extremely strong signals are present in-band. (Usually from very close by transmitters on the same band.)  This mod also requires new K3 f/w, mcu 3.25 or later.

About 09/01/09 (RF board Rev H3)  and later.

Keying the KRX3 SUBOUT Connector  

Your K3 may be damaged if the KRX3 SUBOUT interface board connector is installed with the pins misaligned.

To avoid this possibility, all new K3s have this connector keyed by inserting a plug in one of the holes in J64B and the corresponding pin on all KRX3 SUBOUT boards is cut short.

This modification shows you how to easily install this protective feature in your KRX3 SUBOUT board and RF board.

There is no danger once the KRX3 RF module is properly installed, but if you remove your KRX3 RF module for any reason we strongly suggest you check J64B and ensure it is keyed as described in this document.

About 03/01/09 and later.

Installing Filters in the KRX3, Rev C   

The crystal (roofing) filters in the KRX3 subreceiver are contained in the L-shaped RF module mounted above the main K3 RF board.

Installing or changing the filters involves removing the RF module and the circuit board inside.

The filters plug into the circuit board and are held in place by screws. You will need a Phillips screwdriver and long nose pliers. No soldering is required.

Using the K3 with Elecraft XV Transverters

The Elecraft XV Series high-performance transverters may be used with any transceiver or transmitter/receiver pair that covers the 10 meter band.

This application note describes how to connect your K3 to any combination of these transverters.

Each transverter is selected using the K3 BAND switch and the actual receive and transmit frequencies are shown on the K3 display.

Older K3 Mods and Enhancements - were incorporated into production shipments

KPA3 12V Sense Mod, Rev B

This modification adds a diode to the KPA3 12V sense circuit to hold the voltage to no more than 0.6V when the K3 is turned off. Without it, some K3s refuse to turn off when the POWER button is tapped because the 12V sense circuit does not drop to a low enough level to shut down the K3’s MCU.

The diode is a common leaded part. No work with SMDs is required. A minimum of disassembly is required and no boards need be removed to perform this modification.

Front Panel Microphone Circuit Modification

Some K3 owners have noted distorted transmit audio caused by RF energy being coupled into the front-panel microphone input. In most cases this has been corrected by bypassing an RF choke in the K3 microphone circuit as described in this application note.

Improving the Immunity of the Rear-Panel RS232 and Audio Connectors to RF

Some K3 owners have noted RF pickup problems in lines connected to the K3 rear-panel MIC, PHONES, PC IN/OUT and RS-232 rear-panel connectors. This Application Note describes how to improve the RF grounds on those connectors by bypassing RF chokes in the ground return lines.

Tuning the K3's 60M bandpass filters for some Eu 60M bands

The K3 is supplied with its 60-meter band pass filter adjusted for the USA frequencies. This filter can be readjusted for optimum performance on the 60 meter channels for use in some European countries.

K3 AF Output Mod Kit, Rev C

This modification protects the K3's audio amplifier if either speaker output is connected to a short circuit or very low impedance load. Two 470 ohm, 1/4 watt resistors are added to the output circuit to suppress potentially damaging voltage spikes that might otherwise occur.

The resistors are common leaded parts. No work with SMDs is required to perform this modification.

About 12/23/08 and later.

K3 Synthesizer ALC Modification

The K3's synthesizer has an ALC loop that holds the VCO output relatively constant over a wide range of frequencies and L/C ratios. At high operating temperatures, on some bands, the ALC loop may not provide adequate current to the oscillator transistor, Q1. This can cause the PLL to lose lock, resulting in ERR VCO during VCO CAL, or ERR PL1/ERR PL2 messages on the LCD display when tuning or changing bands.

Reducing the value of resistor R10 from 82 K to 56 K establishes a higher minimum VCO current. This keeps the VCO output above 1.0 V pp even at very high operating temperatures (tested to > 50 C). The result is that the PLL never loses lock.

This modification requires some simple soldering to install a resistor on the KSYN3 board. You may install a conventional leaded 180 K resistor in parallel with the existing SMD resistor R10 or, if you have the tools, you may replace the SMD resistor with a 56K unit. A leaded 180 K resistor can be provided by Elecraft.

The KRX3 subreceiver has a separate KSYN3 synthesizer board. If your K3 is equipped with the KRX3, you should modify both synthesizers.

All K3's shipped on Dec 22, 2008 and later

K3 Negative ALC and Ext Band Data Pull-up Mods, Rev B: REMIOUPGD

This customer mod adds the capability for negative ALC amplifier feedback to the K3.  It also covers adding optional pull-up resistors to the K3 band data digital output lines.

A new KIO3 replacement board, p/n REMIOUPGD: (Rev B Remote I/O (KIO3 Digital))  is now available that includes these changes for $20. (This board replaces the K3 Negative External ALC Mod Parts Kit)   

Most rig / external amplifier combinations do not benefit from hooking up ALC and in fact generate more distortion than without it, due to overshoot from the widely differing ALC voltage, loop gain and ALC reaction times generated by different amplifiers.  Many amplifier manufacturers (Alpha, Acom etc.) recommend not using ALC. 

If you do decide to use ALC, set the K3's power on each band to the level just below where it activates amplifier ALC. (The K3 has a menu option to the remember power level on each band, or globally.) Running the rig at a power much higher than this and relying on ALC from the amplifier to control amplifier drive is a formula for severe distortion and splatter/key clicks. In general ALC should only be used as a fail safe mechanism for those amplifiers that can not tolerate being over driven, need SWR shutdown of the driver etc.


Included on All K3s shipped on Dec 10, 2008 and later. (Rev B KIO3 Digital board)


NOTE: REMIOUPGD is no longer available. The KIO3BUPKT replaces the REMIOUPGD.


(Approx s/n 2362+)

K3 AF Mod kit, Rev B

While the K3's speaker and line outputs meet the original K3 distortion specifications at their intended levels of operation, we've found that further improvement is possible, especially at higher volume levels, with minor component changes. Low level distortion products can be further reduced at both outputs. These changes are being incorporated into new K3s.

The K3AFMDKT modification kit contains the parts to make both speaker amplifier and line-out audio output modifications, but you can choose to do either or both, as they are independent.

Note: The headphone output of the K3, which uses a separate amplifier,  already has a very low distortion output and is not impacted by these changes.

Line Out Change: 

About Oct 7, 2008

About S/N 1900


Speaker Amp Change:

About Sept 25, 2008

About S/N 1830


K3 Front Panel (Rev B) Modification (pdf) 

Incorrect VOX operation (VOX 'hangs') and/or low or corrupted TX audio, when using a front panel mic which requires BIAS to be supplied to the mic through the audio pin. (Elecraft MH2, MD2, Proset-K2 with electret elements etc.) 

Modification involves addition of 1 wire to the rear of the K3 Front Panel board.

Rear panel mic connections do not exhibit this problem. Dynamic microphones do not exhibit this problem. 


Starting approximately April 23-24, 2008

Serial numbers 660 through 782. (As early as S/N 654 may demonstrate this problem, but we think the problem starts with S/N 660.)


Receive Antenna TX Isolation Improvement (RXA Mod Kit for the KXV3)

Some operators have very strong pickup of their transmit antenna signal on their dedicated receive antennas. (Beverages, loops etc.) If the the receive antenna is not externally isolated during transmit, this energy could feed back into the K3 via the KXV3 RX antenna in port. Existing transmit isolation in the KXV3 is sufficient for most installations, but in some cases additional isolation is needed. The RXA Mod Kit for the KXV3 significantly improves isolation. (This change has been made to revision B KXV3 main boards.)

RXA Boards added: Approximately Feb 20, 2008

Rev B KXV3 Main Board added: March 14, 2008

K3 CW Keying Rise Time Reduction

Some owners prefer a faster CW keying waveform rise time. The rise time can be reduced from about 8 milliseconds to 5 milliseconds without affecting the fall time by replacing capacitor C222 on the K3 main RF board with a 0.1 uF capacitor. This change will also allow a future firmware revision to further reduce rise time via a menu entry.

Approximately Jan 20, 2008

Approx. s/n 280

Hardware AGC threshold change (HAGC Mod Kit)

Through hole HAGCMDKT instructions

SMD HAGCMDKT instructions

Both through hole and SMD parts are included. Users can install either set of parts.


Some users prefer tuning the band with a wide filter bandwidth selected (for example, using a 2.7 or 2.8 kHz SSB filter in CW mode rather than a 500 Hz or narrower filter). Strong off-frequency signals (above about S9+5) that are inside the crystal filter passband can then activate the hardware AGC circuit, reducing the level of desired signals. 

This modification raises the hardware AGC threshold from about S9+5 to about S9+25 to +30. The new threshold still protects the DSP's analog-to-digital converter, while providing better receive performance when wider crystal filters are used. This is not as significant an issue when using narrow CW filters (500 Hz and below).


Approximately Jan 20, 2008

Approximately s/n 350

Alert   Click for Document Description (Dates and serial numbers are approximate and may vary slightly.) How do I determine if I already have this ?
K3/100 Rev C KPA3 Spurious TX 11/17/2009: Spurious TX sidebands due to C3 installation on early Rev C KPA3 PC boards. We have determined that a small number of K3/100s and KPA3s left the factory earlier this year (2009) with capacitor C3 installed by mistake on early Revision C KPA3 PC boards. This part location should be empty on Rev. C KPA3s. Installation of C3 causes the PA bias supply to oscillate on some KPA3s at approximately 15-20 kHz, resulting in spurious TX sidebands at that spacing from the carrier. Removal of C3 on Rev C KPA3 PCBs corrects this issue. See Alert Doc. for full details and pictures. A small number of K3/100s within approximately S/N 3000-3138. These radios were shipped approximately April 30 to June 2, 2009. Upgrade or repair KPA3s sent separately during this time period may also be affected. Corrected on all new K3/100s and KPA3s shipped June 2, 2009 and later. See Alert Doc.

KPA3 Power Connector Replacement Manual (pdf)

KPA3 Power Connector Replacement Manual

KBPF3 Mod Kit Instruction (pdf)